ZINC STRESS INDUCES CHANGES IN APOPLASMIC PROTEIN-CONTENT AND POLYPEPTIDE COMPOSITION OF BARLEY PRIMARY LEAVES

Excessive zinc in the rooting medium, and consequently increasing zinc content in the shoot of barley (Hordeum vulgare) seedlings, strongly affected the protein content and the polypeptide composition of the leaf apoplasm. In contrast, only small changes in polypeptide composition (in the IEF range of pH 4-8 and between 14 and 80 kDa) were detected in mesophyll and epidermis protoplasts. The zinc stress-induced changes in apoplasmic protein content were analysed in detail. The apoplasmic protein content increased by more than 3-fold when the zinc concentration in the medium was increased from 0.02 to about 200 mu mol l(-1); at higher zinc concentrations no further changes occurred. The increase in apoplasmic protein content was due both to a general increase in abundance of the most dominant apoplasmic polypeptides and to a pronounced increase in the abundance of specific polypeptides as monitored in electrophoretic separations or by measuring the activity of apoplasmic enzymes. The largest induction was seen for four apoplasmic polypeptides of molecular masses 16, 23, 27, and 28 kDa with as yet unknown function.